Butt-welding plates by plunge stir friction using a rotating post inserted into the joint is known, and described, for example, in U.S. Pat. No. 5,460,317, issued Oct. 24, 1995 in the name of Thomas et al. As described therein, the welding head with its projecting post or probe is pressed against the plates to be welded. This arrangement has several disadvantages, including a tendency to heat portions of the workpieces at regions far from the joint to high temperatures, additional mechanical resistance and wear on the welding machine attributable to the energy going into the heating of regions far from the joint being welded, the cost of the energy required to drive the welding head subject to excess mechanical resistance, and surface deformation of those portions of the workpiece far from the joint. Improved stir weld arrangements are desired.